Reframing neural decussation: A vision-based theory for contralateral control.

Neural decussation, the systematic crossing of sensory and motor pathways at the midline, is one of the most prominent features of the vertebrate nervous system, yet its functional rationale has remained unresolved. This work proposes the Visual-Origin Theory of Neuronal Decussation, which argues that contralateral organization arises as a direct consequence of visual field optical geometry. Light rays from the right visual hemifield physically cross the midsagittal plane before reaching the temporal retina, and so do rays from the left hemifield in the opposite direction. The decussation of nasal retinal fibers in the optic chiasm does not create this crossing; it preserves a pre-existing physical pattern, thereby aligning each cerebral hemisphere with the contralateral half of external space, the only configuration that places a processing center directly "behind" the region of the world it must monitor and act upon. Once hemispheric ownership of visual space is established, motor pathways must follow the same spatial logic. The sensorimotor decussations are therefore the downstream continuation of the same spatial imperative: the shortest, fastest, and most coherent route from a field-specific sensory representation to the effectors acting in that same field is a single midline crossing in the brainstem. A proof-by-exclusion analysis shows that alternative wiring schemes, including fully ipsilateral or reversed crossing patterns, disrupt binocular vision, slow rapid left-right motor responses, and generate visuomotor instabilities observed in clinical disorders of wiring. Comparative evidence across vertebrates, and contrasts with arthropods support the conclusion that neural decussation is an inevitable consequence of vision.